Đáp án:
a) 2x² - 8 = 0
⇔ x = ±2
b) 5x² + 20x = 0
⇔ 5x(x + 20) = 0
⇔ \(\left[ \begin{array}{l}x + 20 = 0\\x=0\end{array} \right.\)
⇔ \(\left[ \begin{array}{l}x=-20\\x=0\end{array} \right.\)
c) 3x² + 5x + 2 = 0
⇔ \(\left[ \begin{array}{l}x=-1\\x=-2/3\end{array} \right.\)
d) y² + 4y + 5 = 0
Δ = 4² - 4.5.1 = 16 - 20 = -4 < 0
⇒ Phuong trinh vo nghiem.